We propose the use of visible-wavelength two-photon excitation (v2PE) for activation of reversibly photo-switchable fluorescent proteins (RSFPs) and successive confocal detection to achieve super-resolution imaging. In this method, three photons interact with the sample molecules in total, which provides imaging properties equivalent to using third-order nonlinearity in fluorescence response. Because this technique uses visible light, it can achieve higher spatial resolution than confocal microscopy. In this study, we performed experimental investigations to confirm the activation of negative RSFPs by v2PE and demonstrated super-resolution imaging of live cells.
Visible-wavelength two-photon excitation (v2PE) is a powerful technique for simultaneous multicolor fluorescence imaging via simultaneous excitation of fluorescent proteins (FPs) with different emission wavelengths. We implemented v2PE into a slit-scanning confocal microscope in order to realize faster simultaneous multicolor fluorescence imaging with utilizing the capability of spectral detection. We demonstrated simultaneous multicolor imaging of living HeLa cells with expressing three types of FPs with different emission wavelengths localized at different intracellular structures. Linear un-mixing of hyperspectral images successfully separated the distribution of multiple FPs expressed in the sample.
KEYWORDS: Microscopy, Luminescence, Spatial resolution, 3D image processing, Confocal microscopy, Two photon excitation microscopy, Stereoscopy, Point spread functions, Objectives, Time lapse microscopy
Two-photon excitation microscopy is one of the key techniques used to observe three-dimensional (3-D) structures in biological samples. We utilized a visible-wavelength laser beam for two-photon excitation in a multifocus confocal scanning system to improve the spatial resolution and image contrast in 3-D live-cell imaging. Experimental and numerical analyses revealed that the axial resolution has improved for a wide range of pinhole sizes used for confocal detection. We observed the 3-D movements of the Golgi bodies in living HeLa cells with an imaging speed of 2 s per volume. We also confirmed that the time-lapse observation up to 8 min did not cause significant cell damage in two-photon excitation experiments using wavelengths in the visible light range. These results demonstrate that multifocus, two-photon excitation microscopy with the use of a visible wavelength can constitute a simple technique for 3-D visualization of living cells with high spatial resolution and image contrast.
For decades, genetically encoded Ca2+ indicators (GECIs), have been under development for the purpose of visualizing intracellular Ca2+ dynamics. Fluorescent GECIs are limited for the purpose of deep tissue long-term whole-body imaging due to their requirement for external illumination. Bioluminescent GECIs overcome these restraints but are somewhat compromised in terms of spatiotemporal resolution compared to fluorescent probes. To address this, we developed a bimodal Ca2+ indicator by combining a single fluorescent protein based Ca2+ indicator and a split luciferase. The novel design of this bimodal indicator enables Ca2+ imaging in the same specimen in both fluorescent and bioluminescent mode. The ability to switch between fluorescent and bioluminescent modes with a single indicator should benefit applications where micro and macro scale observation of cells or tissues is desirable. Use of such a probe enables trans-scale imaging, where macro scale imaging of a group of cells or tissue can be combined with fluorescent imaging of single cells.
Intracellular Mg2+ roles are commensurate with its abundance in the cell cytoplasm. However, little is known about Mg2+ subcellular dynamics, primarily due to the lack of suitable Mg2+-selective tools to monitor this ion in intracellular compartments. To cope with this lack, we developed a Mg2+-sensitive indicator—MagIC (indicator for Magnesium Imaging in Cell) —composed of a functionalized yellow fluorescent protein (FP) variant fused to a red-emitting FP serving as a reference, thus allowing ratiometric imaging of Mg2+. MagIC expressed in mammalian cells is homogeneously distributed between the cytosol and nucleus but its fusion with appropriate targeting sequences redirects it to mitochondria or the endoplasmic reticulum. MagIC shows little interference by intracellular Ca2+ [Kd(Mg2+)=5.1 mM; Kd(Ca2+)=4.8 mM] and its kinetic properties (koff=84 s−1) approach those of indicator dyes. With MagIC, as reported previously, we also observed a cytosolic Mg2+ increase provoked by application of 50 mM MgCl2 in the medium. This effect is, however, mimicked by 75 mM KCl or 150 mM d-sorbitol addition, indicating that it is a response to the associated hyperosmotic shock and not to Mg2+ itself. Our results confirm the functionality of MagIC as a useful tool for the long-awaited possibility of prolonged and organelle-specific monitoring of cellular Mg2+.
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